Currently, with respect to a thin-film transistor (TFT), a wide variety of materials have been searched and considered for use as a channel layer, in an attempt to achieve high performance, low-temperature fabrication process, and low costs. From this perspective, possible materials for the channel layer may be, for example, amorphous silicon, polycrystal silicon, microcrystal silicon and organic semiconductor.
In recent years, as a potent material to be used as such channel layer, an oxide semiconductor has been studied actively. For example, Barquinha et al., J. Non-Cryst. Sol., 352, 1756 (2006) and Yabuta et al., Appl. Phys. Lett., 89, 112123 (2006) disclose methods of fabricating a TFT using an amorphous In—Zn—O (IZO) thin film and an amorphous In—Ga—Zn—O (IGZO) film, being oxide semiconductors for the channel layer.
The TFT will have a different threshold voltage depending on the semiconductor material, fabricating method, etc. of the channel layer. Moreover, the threshold voltage of the TFT will change due to various factors (e.g. manufacturing process history, time-dependent change, electrical stress, thermal stress, etc.). Here, electrical stress is a kind of stress that can be generated by voltage or current as applied to the semiconductor. Thermal stress is a kind of stress that can be generated by heat applied from outside on the semiconductor periphery, or Joule heating of the semiconductor. With the actual TFT, there are cases where such stresses may be applied simultaneously.
The above-mentioned oxide semiconductor TFT is not an exception. For example, Riedl et al., Phys. Stat. Sol., 1, 175 (2007) and Kim et al., International Electron Devices Meeting 2006 (IEDM '06), 11-13, 1 (2006) note that change in threshold voltage due to electrical stress or due to a combination of electric and thermal stresses has been observed. Moreover, with the oxide semiconductor TFT, the threshold voltage can change by irradiating the TFT with visible light and ultraviolet light, as mentioned in Barquinha et al., J. Non-Cryst. Sol., 352, 1756 (2006) and Gorrn et al., Appl. Phys. Lett., 91, 193504 (2007).
Furthermore, Japanese Patent Application Laid-Open No. H10-209460 discloses a method of decreasing the threshold voltage of the TFT, which uses polycrystal silicon as the material of the channel layer, by adopting a structure in that light will enter into the channel layer of the TFT.